Question

Prove the formula A=1/2 r^2 θ for the area of a sector of a circle with radius r and central angle θ. [Hint: Assume 0 < θ < π/2 and place the center of the circle at the origin so it has the equation x^2+y^2=r^2. Then A is the sum of the area of the triangle POQ and the area of the region PQR in the figure.